In hybrid vehicles, electric vehicles, and the like which are actively developed in recent years, a resistance force generated by an electric motor or an electric generator etc. which is driven by a rotation of a wheel is utilized for a regenerative brake. Additionally, even in these vehicles, a hydraulic brake in which a brake device is actuated by a brake fluid pressurized by a master cylinder device is utilized like a conventional vehicle. As for these vehicles, when a brake operation amount is small, that is, when a required brake force is small, the vehicle is braked with the regenerative brake. As the brake operation amount increases with the required brake force increasing, the vehicle is braked with not only the regenerative brake but also the hydraulic brake. Therefore, in the above vehicles, both brakes can brake the vehicle while cooperating with each other such that a brake force in the hydraulic brake is generated in a condition in which a brake force of the regenerative brake is insufficient. For such cooperation, each of master cylinder devices disclosed in the following Patent Literatures is configured to be able to pressurize a brake fluid depending on not a brake operation force by a driver but only a pressure of a brake fluid introduced from a high pressure source. That is, each of the master cylinder devices is configured to be able to voluntarily pressurize the brake fluid irrespective of a brake operation by the driver, thereby actuating the brake device only where necessary.    Patent Literature 1: JP-2007-55588-A    Patent Literature 2: JP-2009-502623-A